ABSTRACT
Likelihood ratios (LRs) are a useful measure of evidential strength. In forensic casework consisting of a flow of cases with essentially the same question and the same analysis method, it is feasible to construct an 'LR system', that is, an automated procedure that has the observations as input and an LR as output. This paper is aimed at practitioners interested in building their own LR systems. It gives an overview of the different steps needed to get to a validated LR system from data. The paper is accompanied by a notebook that illustrates each step with an example using glass data. The notebook introduces open-source software in Python constructed by NFI (Netherlands Forensic Institute) data scientists and statisticians.
ABSTRACT
An automated approach for evaluating the strength of the evidence of firearm toolmark comparison results is presented for a common source scenario. First, comparison scores are derived describing the similarity of marks typically encountered on the primer of fired cartridge cases: aperture shear striations as well as breechface and firing pin impressions. Subsequently, these scores are interpreted using reference distributions of comparison scores obtained for representative known matching (KM) and known non-matching (KNM) ballistic samples in a common source, score-based likelihood ratio (LR) system. We study various alternatives to set up such an LR system and compare them using qualitative and quantitative criteria known from the literature. As an example, results are applied to establish a system suitable for a firearm-ammunition combination often encountered in casework: Glock firearms with Fiocchi nickel primer ammunition. The system outputs an LR and a measure of LR uncertainty. The range of possible LR-values is limited to a minimum and maximum value in areas of the score domain with little reference data. Finally, the feasibility of combining LRs of different mark types into one LR for the entire primer is assessed. For the distribution models considered in this paper, different modeling approaches are optimal for different types of similarity scores. For the chosen firearm-ammunition combination, non-parametric Kernel Density Estimation (KDE) models perform best for similarity scores based on the correlation coefficient, whereas parametric models perform best for the Congruent Matching Cells (CMC) scores, assuming binomial and beta-binomial models for KM and KNM score distributions respectively. Finally, it is demonstrated that individual LRs of different mark types can be combined into one LR, to interpret a set of different marks on the primer as a whole.
ABSTRACT
The electrostatic complexation of rigid poly(p-sulfophenylene-terephthalamide) (S-PPTA), which forms needle-like aggregates in solution, with flexible poly(allylamine hydrochloride) (PAH) resulted in the formation of aligned polymer films. Proton conductivities of a 20:1 S-PPTA:PAH complex membrane were found to be strongly dependent on both the temperature and humidity, reaching a value of approximately 0.2 S cm(-1) at 90 degrees C, which surpasses that of Nafion. In order to investigate the influence of PAH on the structure of S-PPTA films, microscopy and X-ray scattering analysis was conducted. Optical polarization microscopy images were less birefringent for the complexed samples than the pure S-PPTA, which appeared to be the result of a reduction in the size of the ordered domains. However, X-ray analysis revealed that PAH complexation has almost no effect on the direction and degree of the rigid polymer alignment in the film: S-PPTA is aligned homeotropically, where the polymer chains are perpendicular to the surface of the film. Proton conductivity and polymer orientation in the film were essentially found to be independent of the complex composition, up to mole ratios of 10:1 S-PPTA:PAH.
